By regular pumps I assume you mean regular closed-toe ballroom shoes. If so, the decision probably rests on a couple of factors: what kind of dancing you are planning on doing, your current foot strength, and your fashion sense.
Kind of dancing: If you plan to compete, you'll need pumps, preferably tan or nude satin. Many people who compete don't practice in their satin shoes becasue they get dirty easily, but most prefer to practice in shoes similar to those they will be competing in. So if competing is a goal and only one pair of shoes is in the budget right now, I'd go with pumps, tan if you'll be competing soon, black if it's farther down the line. On the other hand, if you plan to do lots and lots of social dancing, practice shoes will definitely be easier on your feet.
Feet: How do your feet feel in heels? If they are not comfortable in everyday wear, they will be worse when you're dancing. You can get pumps with 1 1/2 inch heels, but you usually have to special order them. Most shoes you'll find in local dance shops will be 2 1/2 to 3 inches. Do you have any foot problems that you need to protect? Practice shoes are easier on the feet and can help prevent injury. How is your balance? You'll be learning new ways to balance and if you don't already have good balance, practice shoes are definitely more stable and will make that process easier.
Fashion sense: Will you wear practice shoes outside your lesson? If you plan on social dancing, you'll want to wear your dance shoes to studio parties and whatever other social dance opportunities you'll have. Will you feel comfortable wearing your practice shoes with whatever you will want to wear to the parties?
Whichever you choose, you're getting off on the right foot (sorry, couldn't resist) by getting dance shoes right at the beginning.
Welcome to ballroom dancing! You'll have a great time.
